This invention relates to an inflatable seal for an annular cavity and more particularly to a seal for an annular cavity into which rubber mold material is injected to inspect cylindrical surfaces forming the cavity for cracks and other surface defects. Tre-pan slots or annular cavities are disposed adjacent steam inlet nozzles in both the steam turbine casing and inner cylinders. Such cavities are utilized to provide for rapid temperature changes in the inlet steam nozzle, however these arrangements do not prevent the rapid temperature changes, but merely reduce the stresses produced by these changes. Therefore, the juncture of the nozzle and the casing and inner cylinder is an area which may be subject to temperature induced stress cracking. Thus, it has become necessary to inspect these surfaces for cracks and other surface imperfections. The confined area does not lend itself to visual inspection and rubber magnetic molding material such as M45793HJ manufactured by Dynamold, Inc. is injected into the cavity and allowed to set. The set rubber mold material is then removed and inspected to determine if there are cracks or other surface defects in the steam inlet nozzle or the sleeve adjacent thereto. Herebefore, wooden and brass sleeves have been fitted into the cavity to form a seal so that the magnetic rubber molding material could be injected into the area being inspected, however the wooden and brass sleeves were difficult to fit in the slots so as to form an effective seal, required considerable experience and was very time consuming.